In life science and biology textbooks, you’re likely to come across diagrams that show several organisms connected by branches. These are called cladograms, and they show us how scientists think organisms are related to one another. They are commonly used when studying biological classification and evolution. If you learn to read and interpret these diagrams, they tell the story of organisms’ evolutionary histories.
For Teachers: If you’re an educator planning to teach cladograms or biological classification, great learning resources are key to deepening understanding! You’ll find tons of materials for teaching cladograms and much more in my comprehensive Taxonomy & Classification Unit (plus you’ll support my blog with your purchase! ❤️)
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What are Cladograms?
Cladograms and phylogenetic trees are tools that scientists use to visualize their hypotheses about the evolutionary relationship between taxa. They are diagrams with branches that show how different groups of organisms are related through common ancestors.
Parts of a Cladogram
Cladograms have branches, and each branch ends in a tip representing a taxon (e.g., “cartilaginous fish” or “birds” on the diagram below). Branches come together at nodes. A node represents a hypothetical common ancestor shared by the taxa that branch from it. The cladogram begins at a root, which represents a hypothetical common ancestor of all the taxa in the cladogram.
Reading a Cladogram
Start at the root, which represents a hypothetical common ancestor of all the taxa in the cladogram. As you move across the diagram from the root toward the branch tips, you encounter nodes in order from older to more recent. Nodes closer to the root represent earlier common ancestors, while nodes closer to the branch tips represent more recent common ancestors.
By examining a cladogram, you can compare the evolutionary relationships between taxa. Taxa that share a more recent common ancestor are more closely related than taxa whose most recent common ancestor lived longer ago. For example, in the example diagram below, birds and reptiles share a more recent common ancestor than birds and bony fish do. This means that birds are more closely related to reptiles than to bony fish.
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References and Further Reading
- Hilliard, V. K, Whitenack, L. B., & Ryerson, B. (2025). Comparative Vertebrate and Human Anatomy: Ecology, Evolution, and Function. PALNI Open Press. https://doi.org/10.59319/YHFP2525
- Clark, M.A., Douglas, M., Choi J.,(2018). Biology 2e. Open Stax. Available: https://openstax.org/details/books/biology-2e
- Paleontological Research Institution (2025). Digital Atlas of Ancient Life. Available: https://www.digitalatlasofancientlife.org/
- U.C. Berkley (n.d.). Understanding Evolution. Available: https://evolution.berkeley.edu/